Journal of Physical Chemistry & Biophysics
Biophysics
urnal of Ph Jo
&
l Chemist ica ry ys
ISSN: 2161-0398
Research Article
Wu et al., J Phys Chem Biophys 2015, 5:6 DOI: 10.4172/2161-0398.1000196
OpenAccess Access Open
A DFT Study on the Degradation of Chlorobenzene to p-chlorophenol via Stable Hydroxo Intermediate Promoted by Iron and Manganese Monoxides Yougen Wu1, Yuchen Zhang1 and Yanying Zhao1,2* Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou, China Engineering Research Center for Eco-dyeing and Finishing of Textiles, Key Laboratory of Advanced Textiles Materials and Manufacture Technology, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou, China 1 2
Abstract The reaction paths for the conversion of chlorobenzene to p-chlorophenol are presented in detail using iron and manganese monoxides via the hydroxo insertion intermediate, HO–M–C6H4Cl (M=FeO, MnO). The molecular geometries and electronic structures for the reactants, intermediates, transition states, and products were optimized and analyzed in detail by density functional methods. The reaction potential surface profiles indicate that the metaloxo species can activate the para C–H bond of the chlorobenzene to lead to the p-chlorophenol via the successive formation and the dissociation of the metal carbon bond, followed by removal of the metal atom (Fe or Mn). The intrinsic reaction co-ordinate (IRC) analyses indicated that no crossover point was searched for between the high-spin and low-spin potential energy surfaces; thus, no spin crossing was found between these two states potential energy surfaces. The low-spin potential energy surface lies above the high-spin one for the entire reaction pathway. Our theoretical study on the possible reaction pathways for the conversion of chlorobenzene to p-chlorophenol will also be useful for analyzing the catalytic functions of C–H bond activation and metal–carbon bond formation by transition metal complexes.
Stable hydroxo intermediates formed during the conversion of chlorobenzene promoted by transition-metal monoxides (M represents the Fe or Mn atom).
Keywords: p-chlorophenol; IRC analysis; Hydroxo intermediate; Reaction mechanism; DFT calculations
Introduction Transition metals and their oxides are widely used as both catalysts and catalytic supports for C–H bond activation [1-5]. Nonetheless, these materials have not yet been fully investigated from a comprehensive mechanistic viewpoint [6]. The CH4 + MO+→ M++CH3OH reaction presents one of the simplest and earliest examples of C–H bond activation by transition metal compounds. Prior experimental studies have focused on gas-phase reactions of methane with first-row transition-metal oxide ions [2-10]. Considerable theoretical studies on these reactions have been conducted by Yoshizawa’s group [11-13], who also performed the theoretical study on the reaction of FeO+ with benzene [14,15]. In recent years, Andrew’s group has reacted some of the group IV transition metal atoms with acetonitrile; although the observed experimental product CH2=Zr(H)NC was assigned by matrix isolation infrared spectroscopy and isotopic substituted experiments combing with DFT frequency analysis, the detailed reaction mechanism was not took into account [16]. Further DFT theoretical calculations on the spin inversion process of the reaction pathway were reported by Jin et al. who considered two-state reactivity and spin-forbidden chemical reactions [17]. Besides the reactions of pure transition metal compounds with methane and acetonitrile, some small hydrocarbons such as C2H2, C2H4, and C6H6 [18-20] and halohydrocarbons such as J Phys Chem Biophys ISSN: 2161-0398 JPCB, an open access journal
CH3Cl [21-24] have also been reported. However, the reactions of pure transition metal oxides with halogenated aromatic hydrocarbons have received very little attention. Halogenated benzene compounds, one of the larger groups of anthropogenic materials, are widely used in the chemical and electronics industries. However, most of these compounds are hazardous organic pollutants because of their environmental impact and noxious effects, and are frequently found in various waste oils and other organic liquids. Recently, selective C–H activation with halo [25-27], cynao [28,29], and hydroxo [30-32] functional groups has attracted much attention in organic synthesis due to the possibility of incorporating versatile
*Corresponding author: Yanying Zhao, Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou, China, Tel: +86-571-86843627; E-mail:
[email protected] Received October 08, 2015; Accepted November 25, 2015; Published November 30, 2015 Citation: Wu Y, Zhang Y, Zhao Y (2015) A DFT Study on the Degradation of Chlorobenzene to p-chlorophenol via Stable Hydroxo Intermediate Promoted by Iron and Manganese Monoxides. J Phys Chem Biophys 5:196. doi:10.4172/21610398.1000196 Copyright: © 2015 Wu Y, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Volume 5 • Issue 6 • 1000196
Citation: Wu Y, Zhang Y, Zhao Y (2015) A DFT Study on the Degradation of Chlorobenzene to p-chlorophenol via Stable Hydroxo Intermediate Promoted by Iron and Manganese Monoxides. J Phys Chem Biophys 5:196. doi:10.4172/2161-0398.1000196
Page 2 of 6 functional groups. In these reactions, the functional groups remain unreactive under suitable conditions, thereby allowing the activation of the targeted C–H bonds to form more complicated organic molecules. In addition, hydroxylation of the halogenated benzenes is biologically important, because it forms p-substituted hydroxyhalobenzenes that are mostly found in mosquito larva and water extracts. Recently, group IX metals have been extensively reported for the activation of C–H bonds in the presence of carbon halide bonds. Milstein described an exclusive activation of ortho C–H bonds in chloro- and bromobenzene via the cationic pincer complex [(PNP*)Ir]+ (PNP,′-bis(di--butylphosphino)2,6-diaminopyridine) [33]. Kinetic preference for C–H activation and thermodynamic preference for C–Cl activation for chlorobenzene (PhCl) was observed by Ozerov [34] by using an analogous (PNP)IrI system and also in related DFT calculations by Hall [35]. In this paper, we report a theoretical study of the reactions of neutral MnO and FeO with chlorobenzene, taking spin multiplicities into consideration. The reaction intermediates and the energetics along the reaction pathway are computed and analyzed in detail. Our theoretical analysis on the direct hydroxylation of chlorobenzene will help the researchers in the fields of catalysis chemistry and bioinorganic chemistry.
Computational Methods The computations were performed using the Gaussian 09 ab initio program package [36]. The 6-311++G(d, p) all-electron basis sets were used for all atoms [37,38]. To select an appropriate functional, different functionals (including B3LYP [39-41], M06 [42], M062X [42] and M11 [43]) were tested by calculating the M–O bond lengths and the M–O stretching vibrational frequencies of all first-row transition-metal monoxides (M=Sc to Cu). The results indicated that both wB97XD and B3LYP demonstrated the best results. Since B3LYP is a frequently reported functional, it was chosen to optimize the structures of the molecules under investigation. The first structures to be optimized were the equilibrium structures of PhCl, MnO, and FeO monomers, reactant complexes (PhCl)MO, hydroxo intermediates ClPh(MOH), product complexes, and their transition states with different multiplicities. Complete optimization of the molecular geometries was done with all stationary points. The harmonic vibrational frequencies of all the species were calculated with analytic second derivatives at the same level. This confirms that each stationary point is a local minimum or is a saddle point from systematic vibrational analyses of intrinsic reaction co-ordinates (IRCs) [44,45] and that it evaluates the zero-point vibrational energies (ZPVE). Each transition state was traced from a transition state toward both reactant and product directions along the imaginary mode of vibration using the algorithm developed by Gonzalez and Schlegel [46] in the mass-weighted internal coordinate system. Each IRC was constructed from 50 to 100 steps.
Results and Discussion Potential energy diagram of the reaction between MO and C6H5Cl DFT/B3LYP calculations were performed on the potential reaction products. Figures 1 and 2 shows the computed profiles of the potential energy surfaces for the reaction of transition metal monoxides with chlorobenzene in both the quartet and sextet states for MnO and in both the triplet and quintet states for FeO. Every possible computed structure of all intermediates and transition states is also displayed in Figures S1 and S2 (Provided in supplementary information). From the figures, we conclude that the chlorobenzene–p-chlorophenol J Phys Chem Biophys ISSN: 2161-0398 JPCB, an open access journal
reaction is a two-step reaction: in the first step, the compound passes through a transition state (TS1) to form a hydroxyl intermediate (HOM-C6H4Cl); in the second step, another transition state (TS2) forms that leads to the product. Figure 1 shows that the computed potential energy profiles of the quintet states have lower energy than the triplet states for FeO. Figure 2 indicates that the energies of the sextet states are below that of the quartet states during the entire reaction process. All the triplet states lie above the quintet ones; therefore, we discuss only the quintet states here. The activation energies required to form TS1 from the reactant are 15.7 kcal/mol for MnO and 20.2 kcal/mol for FeO along the sextet and quintet states of the reaction coordinates. Two stable hydroxyl intermediates are also optimized. Notably, no spin crossing was detected for the entire reaction. With the help of B3LYP computation, the overall reaction is predicted to be exothermic for MnO, with the release of 4.4 kcal/mol of energy. In contrast, the reaction is endothermic for FeO, with the required energy being 14.4 kcal/mol.
IRC analyses As shown in Figures 3-5, the quintet-state IRC analysis can be used to investigate the reaction intermediates and transition states for the FeO system. We begin with by investigating the first step-reaction using IRC analysis in which the reactant complex forms the intermediate through TS1. This first step-reaction can be viewed as a 1,3-hydrogen migration (Figure 1). This is the most important step for cleaving p-hydrogen that migrates to oxygen and finally forms a hydroxy intermediate, which combines with OH and C6H4Cl ligands. The first step-reaction in the quintet state has been discussed in detail in Figure 3, in which we present the change in the geometrical parameters during the reaction. The IRC was started from TS1 (s=0), which exhibits a Cs structure with an imaginary vibrational mode of 1678i cm-1, toward both the reactant (s0) directions. In principle, both directions would lead to an energy minimum in the reactant or product “valleys.” Unfortunately, the IRC ended before the true structure of the reactant complex could be acquired, although the terminal energy is very close to that of the reactant complex. However, this IRC analysis is good enough to increase our understanding of the reaction pathway. The most important first step-reaction along the reaction pathway is discussed in detail in Figure 3a, in which the migrating hydrogen FeO+C6H 5Cl triplet 48.5
TS2
TS1
35.6
30.4
0.0
24.3
23.8
15.7 quintet
Fe+HOC6H 4Cl
14.4
14.4
4.1
-14.8 Reactant complex
-0.7 Product complex -15.8
-43.9 Hydroxo intermediate
Figure 1: Potential energy diagram (including zero-point energy) along the reaction pathway, FeO+C6H5Cl→Fe+HOC6H4Cl, in the quintet states. Relative energies are in kcal/mol.
Volume 5 • Issue 6 • 1000196
Citation: Wu Y, Zhang Y, Zhao Y (2015) A DFT Study on the Degradation of Chlorobenzene to p-chlorophenol via Stable Hydroxo Intermediate Promoted by Iron and Manganese Monoxides. J Phys Chem Biophys 5:196. doi:10.4172/2161-0398.1000196
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TS2 M nO+C6H 5Cl
54.3
quartet
TS1
37.4
38.7
41.8 34.4 22.7
20.2 sextet 0.0
M n+HOC6H 4Cl
13.5
4.3
-4.4
-4.4 Product complex
-8.8 Reactant complex
-43.4 Hydroxo intermediate
Figure 2: Potential energy diagram (including zero-point energy) along the reaction pathway, MnO+C6H5Cl→Mn+HOC6H4Cl, in the sextet states. Relative energies are in kcal/mol.
(a)
Next, let us look at the change in the bond angle for the first stepreaction. In Figure 3b, the change in the C–Fe–O angle exhibits an interesting feature: the angle keeps changing and reaches the minimum value (75°) in TS1. The C–H bond begins to dissociate at s=-0.6 and the O–H bond distance is nearly constant (0.96 Å) after passing s=1.5. Therefore, we consider that H-atom migration happens in the range -0.6